fix: reject malformed numeric CLI arguments instead of crashing or coercing to 0

[ayushsingh82]

While looking at how numeric CLI arguments get parsed, I hit two spots in src/utils/ that fail in ways the caller can't see coming.

asBigInt handles deposit ids, intent amounts, maxCost and durations across the deposit / intent / stake / vault / delegate commands. Pass it a hex, fractional, or non-numeric string like "0x10", "1.5" or "abc" and it throws a bare SyntaxError; a non-integer number throws a bare RangeError. Neither goes through the VALIDATION_ERROR envelope the rest of the CLI uses, so the user just gets a stack trace. Worse, BigInt("") is 0n, so a blank --deposit-id quietly becomes deposit 0 with no error at all.

amountToUnits has a similar sharp edge. An amount smaller than one base unit gets rounded down to 0n by parseUnits, so a transfer or approve silently turns into a no-op. And Number#toString switches to exponential notation for very small or very large magnitudes (1e-7, 1e+21), which parseUnits rejects outright.

The change keeps valid input behaving exactly as before:

• asBigInt now accepts only a plain decimal-integer string or a safe-integer number. Anything else returns a VALIDATION_ERROR that names the field and shows the value it choked on. The bigint passthrough is untouched.
• amountToUnits rejects a sub-base-unit amount and an implausibly large one with a clear message, and formats through toFixed(decimals) so parseUnits always sees plain decimal notation.

Checked:

• asBigInt — "", "0x10", "1.5", "abc", 1.5, NaN and 2**53 are each rejected with a VALIDATION_ERROR; 42, "42" and " -7 " still parse fine.
• amountToUnits — 0.0000001, "0.0000004" (would have rounded to 0n) and 1e21 are each rejected; 0.000001 still returns 1n.
• npm run check is green: lint, typecheck, 234 tests, and coverage with parsing.ts and validation.ts at 100% lines.

[ayushsingh82]

@ADWilkinson small correctness-only fix — no behaviour change on valid input, just turns two silent-failure paths (blank --deposit-id → deposit 0, sub-unit amount → 0n no-op) and two raw-exception paths into clean validation errors. npm run check green. Happy to adjust scope.